The present invention relates to focus detecting devices for cameras. More specifically, the invention relates to a focus detecting device in which focus detection is carried out by detecting the contrast of an image formed by the photographing lens.
It is well known in the art that a characteristic curve indicating the relation between an amount of defocus (defined as the distance between the image plane in a focused condition and the image plane in its defocused condition) and the amount of high frequency components in the spatial frequency spectrum of the image has a peak value as shown in FIG. 1 at the focused position and that, as the amount of defocus increases, the amount of high frequency components correspondingly decreases. Furthermore, a high frequency component detecting technique is also known in the art in which an array of light receiving elements having uniform photoelectric characteristics are arranged in a line as shown in FIG. 2 in a plane parallel to the image plane. The amount of high frequency components is detected by forming the sum of differences between the outputs of adjacent light receiving elements. In FIG. 2, five light receiving elements are arranged in a single straight line. The amount of high frequency components detected in this manner will be referred to as "a contrast output" for simplification of description hereinafter.
In some conventional focus detecting devices, sensors are disposed on both sides of the film surface to detect a focusing condition according to these principles. An example of a conventional light beam splitting arrangement is as shown in FIG. 12 in which a light beam is split by a half-silvered mirror 14 into two separate beams which are applied to sensors 3 and 5. However, it should be noted that, with the arrangement in FIG. 12, it is difficult to provide the two sensors on a single substrate. Accordingly, it is also difficult to make the characteristics of the two sensors uniform or to align the positions of the half-silvered mirror and the two sensors with the result that it is difficult to miniaturize such a focus detecting device.
Shown in FIG. 13 is another example of a conventional light beam splitting arrangement. In this example, a light beam reflected by a half-silvered mirror 14 is deflected parallel to the light beam passing through the half-silvered mirror 14 by the provision of a mirror 15. In this case, it is possible to provide two sensors on one substrate. However, the arrangement is disadvantageous in that it is difficult to firmly position the half-silvered mirror 14, the mirror 15 and the sensors 3 and 5 and accordingly such a focus detecting device is necessarily bulky.
An object of the invention is to provide a focus detecting device in which two arrays of contrast detecting light receiving elements, hereinafter referred to as "sensors", are disposed on both sides of the film surface of a camera and the contrast outputs of the sensors are compared thereby to detect focusing conditions such as front focus, correct focus, and rear focus.